The manufacture of tofu generally involves steps, such as the adding and mixing a coagulant to soy milk, under gently stirring at a temperature of 70 to 75.degree. C., charging the mixture in a molding container, removing a product from the container, cutting and immersing it in water. Soy milk, meanwhile, is usually manufactured by soaking soybeans in water for 12 to 17 hours, then grinding the soybeans along with water to obtain go, boiling this go to subject it to a heat treatment, and then separating the lees of soybean from this go by centrifugation, pressing, or another such means.
In the past, the soy milk separated from the heated go would undergo a sharp increase in viscosity during a tofu manufacturing process such as this, and this is known to make handling of the soy milk extremely difficult. In an effort to improve aptitude for the secondary processing of soy milk, attempts have been made in the past at lowering the viscosity of the soy milk obtained by separating from the go, or of this soy milk to which a coagulant has been added.
For example, Japanese Laid-Open Patent Application 1-218567 (1989) discloses a method for keeping the low level of the viscosity of the soy milk to which magnesium chloride is added at a concentration of 13% or less, wherein the soy milk is prepared by heating indirectly the go solution obtained by grinding raw soybeans up to 100.degree. C. to 105.degree. C. under pressure to agitate and uniformly denature it, then separating the lees of soybean, and cooling the product. Also, Japanese Patent Publication 48-34225 (1973) discloses a method for remarkably decreasing the viscosity of a protein-dispersed solution in which, in the process of treating protein obtained from soybeans, the protein-dispersed solution being adjusted to a neutral pH is treated in a high-pressure homogenizer, heated to at least 120.degree. C. by the introduction of live steam in a pipeline, held at this temperature for no longer than 10 seconds to undergo specific sterilization and denaturation, and this product is then sprayed into a vacuum chamber and thereby concentrated and cooled to obtain the product.
With the former method, however, the go solution must be indirectly heated under pressure up to the product temperature of 100 to 105.degree. C., and an apparatus accomplishing this indirect heating under pressure is needed. Similarly, with the latter method, the protein-dispersed solution must be treated in a high-pressure homogenizer, and then must be heated to at least 120.degree. C. by the introduction of live steam in a pipeline, and must be held at this temperature for no longer than 10 seconds, so a high-pressure homogenizer and equipment for introducing the live steam into the pipeline are needed. Also, in the latter case, the object of the method is to enhance the sterilization and digestion of a soya protein product, denature the properties of it, and enhance the storage stability of the finished product, and the manufacture of tofu is not targeted in the case.